Natural Vacuum Leak Detection (NVLD) Small Leak Monitor

The engine off NVLD small leak monitor is executed by the individual components internal to the NVLD module as follows:

1. The PCM uses inputs from the ECT sensor, the IAT sensor, the MAF sensor, the VSS and the NVLD module to determine conditions of the enhanced EVAP system. The combination of these signals are used by the PCM to determine when to activate the EVAP leak check monitors.
2. When the ignition is turned OFF and the calibrated conditions are met the PCM sends a message to the NVLD module to begin the engine off NVLD monitor.
3. The EVAP canister purge valve is normally closed with the ignition OFF.
4. The NVLD leak check monitor uses the naturally occurring change in fuel tank pressure as a means to detect a leak in the EVAP system.

   The small leak check monitor is controlled by a separate low power consuming microprocessor inside the NVLD module.

   The small leak check monitor is executed with the ignition OFF, after the engine running EVAP leak check monitor is completed. The heat generated while the engine is running warms the fuel in the fuel tank. When the engine is turned OFF a natural vacuum is generated by the fuel cooling in the fuel tank. On a normally operating EVAP system this vacuum closes the NVLD vacuum switch. The NVLD vacuum switch position is checked after 10 minutes from engine shut down. If the NVLD vacuum switch is closed the small leak monitor passes.

   If, after 10 minutes from engine shut down the NVLD vacuum switch is not closed, and the NVLD ambient air temperature sensor change is more than 8°C (14°F) over the next 24 hour period without the vacuum switch closing, the test fails.

   The PCM receives a message from the NVLD module at ignition ON and then engine running, indicating the EVAP system has either passed or failed the small leak test.

   A vacuum decay test is executed as a rationality test to the NVLD small leak check monitor. The vacuum decay rate is determined by the calculated fuel tank pressure, leak size, the fuel tank fill level, the NVLD ambient temperature sensor, and the fuel type. Tank pressure is determined by tank fill level, AAT sensor, the EVAP canister purge valve opening, and NVLD vacuum switch position prior to engine OFF. If either the NVLD small leak check monitor passes or the vacuum decay test passes the PCM considers the EVAP system passed the leak test. If the NVLD small leak check monitor fails and the vacuum decay test passes the PCM considers the EVAP system passed the leak test

5. On a normally operating EVAP system, a vacuum is generated inside the fuel tank as the temperature of the fuel decreases.

Fig 1: EVAP NVLD Small Leak Check Monitor System Diagram

Courtesy of FORD MOTOR CO.